// es5 构造函数
// 函数表达式
const MiniStack = function() {
    this.stack = [];    //数组
}
MiniStack.prototype.push = function(x) {
    this.stack.push(x);
}
MiniStack.prototype.pop = function(){
    return this.stack.pop()
}
MiniStack.prototype.top = function() {
    if(!this.stack || this.stack.length) {
        return ;
    }
    return this.stack[this.stack.length-1];
}
// O(n)
MiniStack.prototype.getMin = function (){
   
}